474
ERIC E. CONN
This system consists of reactions catalyzed by four enzymes: a dehydrogenase requiring TPN
+ , glutathione reductase, dehydroascorbic acid reductase, and ascorbic acid oxidase (236, 237). Reactions 21-24 are
catalyzed by these four enzymes, respectively:
Substrate + TPN+ ^± Oxidized substrate + TPNH + H+
(21)
TPNH + H
+ + GSSG ?± TPN+ + 2 GSH
(22)
2 GSH + DHA ^± GSSG + AA
(23)
AA + i 0 2 -* DHA + H 2 0
(24)
The oxidation of ascorbic acid (Reaction 24) can also be mediated
through the phenol oxidases or cytochrome oxidase.
Although a fraction of the respiration of the pea seedling may be
mediated through the respiratory chain described in Reactions 21
through 24, the question of the physiological significance of this respiration has been raised. The possibility of these oxidations being associated
with phosphorylation is questionable, inasmuch as the oxidative enzymes
are found mainly in the soluble fractions of the cell. If the oxidationreduction potential (E 0 ) of the GSSG-GSH couple is +0.03 volt (238),
the difference in potential between glutathione and the pyridine nucleotide is large enough to support phosphorylation in Reaction 22. However, attempts to demonstrate oxidative phosphorylation during the
anaerobic reduction of oxidized glutathione by Krebs cycle substrates in
the presence of avocado mitochondria were unsuccessful (239). Results
such as these suggest that the function of the respiratory chain studied
by Mapson and Moustafa is other than making energy available to the
plant. There is apparently only one report of an energy-consuming
process, that of ion absorption, being linked to ascorbic acid oxidase
(240). Reactions 21 through 23 provide a means for maintaining glutathione and ascorbic acid in the reduced state. Since reduced glutathione
is known to influence the enzyme activity of proteins containing sensitive sulfhydryl groups, this may be the major function of the enzyme
system which accomplishes reduction of glutathione in living tissues
(236,237).
There is another way in which the oxidation of ascorbic acid can be
linked to the oxidation of a reduced pyridine nucleotide. Several years
ago Mathews (241) observed that a protein fraction from pea seeds
catalyzed the oxidation of DPNH in the presence of ascorbic acid. Since
the reaction was insensitive to cyanide, Mathews concluded that
ascorbic acid oxidase was not involved in his studies. Subsequent work
has shown that peas (242, 243), yeast (244), and other plants (243) contain an enzyme which will catalyze the oxidation of DPNH by a product
formed by the action of 0 2 on ascorbic acid in the presence of its oxidase (Reaction 24). Although dehydroascorbic acid is the final oxidation
product when ascorbic acid is oxidized in the presence of its oxidase.
ERIC E. CONN
This system consists of reactions catalyzed by four enzymes: a dehydrogenase requiring TPN
+ , glutathione reductase, dehydroascorbic acid reductase, and ascorbic acid oxidase (236, 237). Reactions 21-24 are
catalyzed by these four enzymes, respectively:
Substrate + TPN+ ^± Oxidized substrate + TPNH + H+
(21)
TPNH + H
+ + GSSG ?± TPN+ + 2 GSH
(22)
2 GSH + DHA ^± GSSG + AA
(23)
AA + i 0 2 -* DHA + H 2 0
(24)
The oxidation of ascorbic acid (Reaction 24) can also be mediated
through the phenol oxidases or cytochrome oxidase.
Although a fraction of the respiration of the pea seedling may be
mediated through the respiratory chain described in Reactions 21
through 24, the question of the physiological significance of this respiration has been raised. The possibility of these oxidations being associated
with phosphorylation is questionable, inasmuch as the oxidative enzymes
are found mainly in the soluble fractions of the cell. If the oxidationreduction potential (E 0 ) of the GSSG-GSH couple is +0.03 volt (238),
the difference in potential between glutathione and the pyridine nucleotide is large enough to support phosphorylation in Reaction 22. However, attempts to demonstrate oxidative phosphorylation during the
anaerobic reduction of oxidized glutathione by Krebs cycle substrates in
the presence of avocado mitochondria were unsuccessful (239). Results
such as these suggest that the function of the respiratory chain studied
by Mapson and Moustafa is other than making energy available to the
plant. There is apparently only one report of an energy-consuming
process, that of ion absorption, being linked to ascorbic acid oxidase
(240). Reactions 21 through 23 provide a means for maintaining glutathione and ascorbic acid in the reduced state. Since reduced glutathione
is known to influence the enzyme activity of proteins containing sensitive sulfhydryl groups, this may be the major function of the enzyme
system which accomplishes reduction of glutathione in living tissues
(236,237).
There is another way in which the oxidation of ascorbic acid can be
linked to the oxidation of a reduced pyridine nucleotide. Several years
ago Mathews (241) observed that a protein fraction from pea seeds
catalyzed the oxidation of DPNH in the presence of ascorbic acid. Since
the reaction was insensitive to cyanide, Mathews concluded that
ascorbic acid oxidase was not involved in his studies. Subsequent work
has shown that peas (242, 243), yeast (244), and other plants (243) contain an enzyme which will catalyze the oxidation of DPNH by a product
formed by the action of 0 2 on ascorbic acid in the presence of its oxidase (Reaction 24). Although dehydroascorbic acid is the final oxidation
product when ascorbic acid is oxidized in the presence of its oxidase.